Abstract

The discovery of Ziegler-Natta (ZN) catalysts for the polymerization of ethylene and propylene revolutionized the plastics industry. The first generation of catalysts for isotactic polypropylene was based on TiCl3 (as such or generated in situ). It was later discovered that improved catalysts could be obtained by generating the active Ti species on a support of MgCl2. This way, a much higher percentage of the titanium is active and the use of various donors and additives allows more precise control of stereochemistry, comonomer incorporation, etc.
A standard procedure for generating such supported ZN catalysts consists of ball-milling together the MgCl2 support, TiCl4 and an internal donor, then removing the excess TiCl4 and donor by washing, and finally activating the catalyst using a mixture of aluminium alkyl and an external donor. Catalysts prepared in this way typically contain 1-2% Ti by weight. The accepted "picture" of this catalyst preparation is that TiCl4 adsorbs strongly on certain sites of the MgCl2 surface (possibly modulated by the internal donor), and then is firmly bound at those sites during subsequent washing, activation and catalysis. If this is the case, unambiguous knowledge of which MgCl2 surface sites can bind TiCl4 strongly would lead to understanding of the location and environment of active surface sites.
As the first step of a thorough computational modelling of these systems, we studied the bulk and surface structure of the ordered α and β phases of MgCl2 by means of periodic DFT (B3LYP) methods using localized Gaussian basis sets. The layer structure of both phases was reproduced satisfactorily with the inclusion of a (small) empirical dispersion correction (‘‘DFT-D’’) as a practical method to describe the attraction between the layers. Surface models were studied on slabs with adequate thickness. It appears that various surfaces exposing 5-coordinated Mg are very close in energy and are the lowest non-trivial surfaces. Cuts exposing 4-coordinated Mg are significantly less stable; both kinetic and equilibrium models of crystal growth indicate that they should normally not be formed to a significant extent. ‘‘Nano-ribbons’’ of single, flat chains of MgCl2, sometimes proposed as components of the disordered δ phase, were also evaluated, but are predicted to be unstable to rearrangement.
Understood which MgCl2 surfaces were exposed, we studied the adsorption of TiCl4 on them. The TiCl4 binding energy to various MgCl2 surfaces was calculated using periodic DFT methods. Calculated values are surprisingly sensitive to the choice of basisset and functional; this may in part explain the large spread of values reported in the literature. A basisset of at least TZVP quality is needed on Cl to avoid overestimation of the binding energy; basissets on Mg and Ti are less critical. In addition, dispersion corrections appear to be significant. Making assumptions that probably err on the side of stronger bonding, we still find that TiCl4 binding to MgCl2 is too weak to be compatible with the classical picture of Ziegler-Natta catalyst formation by irreversible TiCl4 adsorption to MgCl2 crystal surfaces.
Then we considered the adsorption of electron donor molecules as water, ammonia, methanol, mono- and di-metoxyethers, mono- and di-metoxysilane on selected MgCl2 surfaces. For these adsorptions B3LYP DFT method was chosen and a basisset of DZVP quality was adopted for H, C, N, O and Si. For simple systems (CO, H2O, NH3) also basisset TZVP was adopted but, compared with DZVP, no significant differences were found. The result was that donors adsorbed strongly on the surface with unsaturated Mg atoms, poisoning all MgCl2 surfaces. A hypothetical reaction of TiCl4 with donors before adsorption process was taken in account and, for some donors, estimated as exergonic process. CH3OTiCl3 was one product of this reaction, it was found binding to MgCl2 surfaces better than TiCl4 molecule.
Finally, starting from surfaces where, in accord with our calculations, Ti is adsorbed ((110)/TiCl4 and (110)/TiCl3OCH3), we modelled, by cluster approach, ethylene and propylene catalysis, focusing on insertion and β Hydrogen Transfer. On the basis of the energies involved in this process we obtained predictions on molecular mass of polymer.
The comparison between these two catalytic systems in ethylene polymerization, was really interesting, in particular the molecular mass prediction for the latter case is in fair agreement with the experimental values.
Then we also checked the substitution of CH3O group with other electron donors as OH or NHCH3 and we obtained similar results.
Our computations suggest that catalytic sites can derive from TiCl4 adsorbed on (110) surface but only after that TiCl4 reacted with donor as, for example, water, methanol, ethers, ammine etc.